Crystal Chemistry, Elastic Properties and Melting Behaviors of Iron-Bearing Materials in Earth and Planetary Interiors.

Abstract

Iron (Fe), with the maximal nuclear stability, is the most abundant element in the Earth. With a partially filled 3d shell, iron adds a variety of influences onto the physical properties and chemical behavior of the Earth and planetary interiors. This dissertation addresses a number of issues concerning the nature and dynamics of the Earth lower mantle and the terrestrial cores. In research chapter I, I studied the crystal chemistry of lower mantle bridgmanite by temperature dependent Mössbauer spectra. I inferred the site occupancy of Fe2+ and Fe3+ in a large number of bridgmanite samples reported in the literature and offered new insights into the crystal chemistry, spin states and electric conduction mechanisms in bridgmanite. In research chapter II, I measured lattice parameters of Fe7C3 at 300 K and up to 68 GPa. Two discontinuities were found in the compression curve, which can be attributed to ferromagnetic to paramagnetic transition and paramagnetic to nonmagnetic transition, respectively. By extrapolating the established equations-of-state to the inner core pressure-temperature conditions, the nonmagnetic phase provides a good match for the observed density of the inner core, whereas the paramagnetic phase would be denser by 12~13 %. In research chapter III, I proposed a new mechanism to explain the nonubiquitous occurrence of ultra-low velocity zones based on my eutectic melting curve of Fe-C system: About 1 wt.% mixture of iron and diamond/iron carbide could be generated in subducted oceanic lithosphere when penetrating the 660 km discontinuity. Such mixture would melt in the basal part of the lower mantle and significantly lower seismic velocities. In research chapter IV, I investigated the melting behaviors of iron-nickel-sulfur system at the pressure of the lunar inner-core boundary (5.1 GPa). Based on my data, I proposed that lunar core with 5.4 ~ 11.3 wt.% sulfur would start to crystallize in the center and support a long-lived lunar dynamo; due to the growth of the inner core, the crystallization of such lunar core would switch to take place at the top of the core and the lunar dynamo is expected to shut down shortly afterwards.